Yes, I know. Any changes made to the custom.css with div-styling that are used in ACP as well are changed. This includes the body-tag, buttons, and others. Custom.css takes priority over every css file. When adding a new theme as default, it is by default in the admin panel too. You can change it back to the actual default one though by clicking on "Signed in as..." and "Change Theme >". The style you used for the front side, is used for the admin panel too.